Photographer Arko Datto's pictures of people sleeping in the open in the eastern Indian city of Calcutta.

Tens of thousands of people sleep at night on the streets of India's eastern city of Calcutta. Arko Datto has been taking photographs of people sleeping in the open in the city's College Street area, well known for its cheek-by-jowl bookshops.

Among the homeless people sleeping on the streets are migrant workers from neighbouring states. Mr Datto says he was drawn to their plight when, as a student of a prominent college in the area, he campaigned to raise funds and distribute blankets for the sleepers in the winter.

"The din of the city during the day gives way to a eerie stillness at night and the colourful bookshops lining the busy street become a temporary refuge of the migrants," says Mr Datto.

Most of the sleepers have left their families to come and find work in Calcutta. Mr Datto says he began thinking of documenting the sleepers after working on a story on migrant workers in the mountainous Ladakh region of Indian-administered Kashmir.

Sleeping in the open saves them the cost of renting a place. It is difficult for most of them to afford accommodation with their low wages even in a city where rents are relatively cheap. "I find the differences in 'colour spaces' and the postures they sleep in very interesting," says Mr Datto.

Many of them wake up early in the morning to go to find work as labourers in the city.

Some have their own handcarts to ferry goods from one place to another.

The book stores, Mr Datto says, are "perhaps the last remaining remnants of the quaint old charm that Calcutta was known for".

The 2011 Census estimated 70,000 homeless living in Calcutta, up from 55,000 in 2001. NGOs say the actual numbers could be much higher.